The Role of Iron-Bearing Minerals in NO₂ to HONO Conversion on Soil Surfaces

Abstract

Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO2 on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO2 reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO2 by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO2 hydrolysis mechanism. The magnitude of NO2-to-HONO conversion depends on the amount of Fe(2+) present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO2 and Fe(2+)(aq) present in thin films of water coating the surface, whereas in the range of pH 5-8 HONO stems from reaction of NO2 with structural iron or surface complexed Fe(2+) followed by protonation of nitrite via surface Fe-OH2(+) groups. Reduction of NO2 on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO2] ratios observed during dust storms in urban areas.

Isotopic constraints on wildfire derived HONO

“…solved under this mechanism (0.24 to 0.72) is at least twice as large as values derived from the previous field measurement of aged wildfire smoke (Baylon et al, 2018). By contrast, inclusion of R7 in addition to R6 in rate ratios 3:1 and 20:1 based on previous lab studies (Kebede et al, 2016;Scharko et al, 2017) and are consistent with the limited field measurements (Parrington et al, 2013;Baylon et al, 2018), and further indicate the important role peroxy radicals play as an oxidant in wildfire smoke impacted environments.…”

“…where 2 ̅̅̅̅̅̅̅ is the mean NO2 molecular velocity, S/V is the surface-to-volume ratio of particles, which could range from 9.0 × 10 -6 cm 2 /cm 3 to 3.0 × 10 -4 cm 2 /cm 3 for normally polluted areas and highly polluted areas respectively (Spataro and Ianniello, 2014). The S/V in biomass plume has huge uncertainty; additionally, ground surface may also play a role in nighttime HONO production (Scharko et al, 2017;Kebede et al, 2016;Stemmler et al, 2006), however, its S/V is not well defined/quantified.…”

“…particulate matter (R7) (Scharko et al, 2017;Kebede et al, 2016;Martins-Costa et al, 2020). While the emission sources and heterogeneous pathways were hypothetically used to account for missing HONO sources (Stemmler et al, 2006;Su et al, 2011;Ye et al, 2016;VandenBoer et al, 2014;Donaldson et al, 2014a;Kebede et al, 2016;Scharko et al, 2017), their relative importance is poorly quantified due to large uncertainties associated with emission heterogeneity, surface area and composition, environmental condition (day versus night, temperature, relative humidity), quantification of heterogeneous reaction rate and knowledge gaps in detailed mechanisms. As a result, the HONO budget in wildfire smoke plumes remains poorly constrained.…”

Nitrogen cycling microbiomes are structured by plant mycorrhizal associations with consequences for nitrogen oxide fluxes in forests

“…This in turn lowers the abundance of available ammonia, nitrifiers, and nitrification rates, which are all directly related to NO y flux. It was surprising that soil clay content was a significant factor in this model due to its lack of correlation to ECM tree abundance (Figure S5); however, clay content is likely important in modulating oxic and anoxic conditions, soil porosity, and soil reactivity which may influence NO y production (Kebede et al., 2016; Maharjan & Venterea, 2014). Furthermore, the amount of variance explained increased from 23% in the initial model (Mushinski et al., 2019) to 39% for fixed effects only and 80% for fixed effects plus the influence of geographic location (mixed model, this paper).…”

Effect of NO&lt;sub&gt;X&lt;/sub&gt;, O&lt;sub&gt;3&lt;/sub&gt; and NH&lt;sub&gt;3&lt;/sub&gt; on sulfur isotope composition during heterogeneous oxidation of SO&lt;sub&gt;2&lt;/sub&gt;: a laboratory investigation

“…When NOX and SO2 coexisted, the content of sulfite on the surface of all oxides was reduced significantly (Pan et al, 2019). Simultaneously, HONO was formed by NO2 and subsequent hydrolysis in thin films of water coating boundary layer surfaces according to reaction (R1) (Kebede et al, 2016). The release of HONO may help to sustain the efficient sulfate production and droplet acidity.…”